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Plant room / HIUs
Heat interface units
It has no flue, no burner and nothing to condemn. It still gets the call-out.
A heat interface unit is the connection between a heat network and one dwelling. It takes hot water from the network primary and makes that dwelling’s heating and its domestic hot water. The two circuits never mix, and the unit meters what was used. It replaces a boiler and it is nothing like one: no combustion, no flue, no gas safety regime. What it does inherit is being the only part of the system the resident can see.
Inside
What is actually in the box
One or two plate exchangers
A brazed plate exchanger makes the domestic hot water instantaneously as it is drawn. An indirect unit has a second exchanger for the heating circuit; a direct unit feeds the heating from the primary without one.
A control valve and its actuator
The primary flow is throttled to hold the hot-water outlet at setpoint, reacting within seconds of a tap opening. That valve and its actuator are the part that does the work and the part that fails.
A strainer and a meter
A small filter protects the plate channels from anything the network carries, and a heat meter records what the dwelling used. Both are service items, and the filter is the first thing to look at on almost any complaint.
Instantaneous or stored
The design choice behind the box
Instantaneous
An instantaneous unit stores no hot water. It makes it on demand, so it has to be sized for the peak draw of that dwelling. The network then has to deliver that peak the moment somebody asks for it.
With storage
A small cylinder inside or beside the unit covers the peak, so the primary demand is smoother and smaller. It costs space in a flat, which is usually the argument against it.
Keep-warm
Instantaneous units often hold the exchanger warm so hot water arrives quickly. It is a comfort feature with a standing cost. Keep-warm draws from the primary continuously, and across hundreds of dwellings it is a measurable part of a network’s summer load.
No hot water
And why the unit is usually not the cause
The primary is not arriving hot enough
If the network flow temperature at that riser has drifted down, the unit cannot make setpoint however well it is working. This is a network fault presenting as an appliance fault.
The strainer is loaded
Restricted primary flow looks exactly like a failed unit from inside the flat. It is a five-minute check and it should be the first one.
The control valve or actuator
A valve that has stopped modulating gives either no heat or scalding output. This one genuinely is the unit.
The plate exchanger has scaled
In hard-water areas the domestic side scales, and output falls off gradually rather than suddenly. It is the same approach-temperature story as any other exchanger, in a smaller box.
Servicing
What is on the list
Filter, exchanger, actuator, meter
Clean the strainer, check the domestic exchanger for scaling, confirm the actuator still modulates through its range, and verify the meter. There is no combustion check and no flue, so the visit is short if access is straightforward.
Performance is set upstream
A unit servicing perfectly on a network with a drifted flow temperature will still not satisfy the flat. Read HIU complaints as network data rather than as appliance data.
And the energy centre still has to be maintainable
Everything above depends on plant that runs continuously and gets worked on while it runs. Insulation in a network energy centre therefore has to come off and go back on.
Reference
No hot water: where to look, in order
| What to check | What it means | Whose fault it is |
|---|---|---|
| Network flow temperature at the riser | Drifted below what the unit needs | Not the unit |
| Primary strainer in the unit | Loaded up, restricting flow | Five-minute check |
| Control valve and actuator | Stopped modulating | This one is the unit |
| Domestic plate exchanger | Scaled, output falling gradually | Hard-water areas |
| Meter and controls | Reading or configuration | Billing complaint rather than heat complaint |
What to check · What it means · Whose fault it is
Asked about heat interface units
Questions.
Answers distinguish modelled heat-loss estimates from equipment selection and service guidance.
How does a heat interface unit work?
It takes hot water from the heat network primary. A plate exchanger makes the dwelling’s domestic hot water instantaneously as it is drawn. A control valve throttles the primary to hold the outlet at setpoint. An indirect unit has a second exchanger for the heating circuit. A meter records what was used. Nothing burns and there is no flue.
What is the difference between a direct and an indirect HIU?
An indirect unit has a heat exchanger between the network primary and the dwelling heating circuit, so the two are hydraulically separate. A direct unit feeds the heating straight from the primary. Both make domestic hot water through an exchanger; the difference is on the heating side.
Why has my heat interface unit got no hot water?
Most often something upstream of it. The network flow temperature at that riser has drifted, or the strainer inside the unit has loaded up and is restricting primary flow. If both are fine, the control valve or its actuator has stopped modulating, or the domestic plate exchanger has scaled.
What is keep-warm on an HIU?
The unit holding its domestic exchanger warm so hot water arrives at the tap quickly instead of after a long draw-off. It is a comfort setting with a standing cost, because it takes heat from the primary continuously. Across a whole network that is a measurable part of the summer load.
What does servicing an HIU involve?
Cleaning the primary strainer, checking the domestic exchanger for scaling, confirming the control actuator still modulates through its range, and verifying the meter. There is no combustion or flue check, so it is a short visit where access is straightforward.
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